Analytical Chemistry

Table 4.7 Typical SPE sorbents and interaction mechanisms

Sorbent Polarity Interaction mechanisms

silica, SiO 2

Florisil, MgSiO 3 adsorption;

H-bonding

alumina, Al 2 O 3

Bonded phases (modified silica)

—C 18 H 37 (C 18 or ODS)

—C 8 H 17 (C 8 or octyl) van der

Waals'

interactions

and π–π

interactions

—C 6 H 5 (phenyl)

—(CH 2 ) 3 CN (cyanopropyl)

—(CH^ polar interactions; H-bonding

2 ) 3 NH 2 (aminopropyl)

—(CH 2 ) 3 C 6 H 4 SO 3 H ionic cation exchange

—(CH 2 ) 3 N(CH 3 ) 3 Cl ionic amion exchange

chiral (cyclodextrin) polar adsorption; H-bonding; dipolar

interactions; steric effects

styrene/divinyl benzene copolymer non-polar size exclusion

graphic sorption mechanisms (p. 80) can be exploited depending on the sorbent selected and the nature
of the sample. Thus, hydrophobic and van der Waals' forces, dipolar interactions, H-bonding, ion-
exchange and exclusion may contribute to or predominate in the retention of sample components, mixed
mechanisms being common. Figure 4.8 represents these interactions diagrammatically for a number of
sorbent surfaces and solute functionalities, some typical applications being given in each case. Sorbents
can be classified according to the polarity of the surface. The hydrocarbon-modified silicas are non-
polar, and therefore hydrophobic, but are capable of extracting a very wide range of organic compounds
from aqueous solutions. However, they do not extract very polar compounds well, if at all, and these are
best extracted by unmodified silica, alumina or Florisil, all of which have a polar surface. Ionic and
ionizable solutes are readily retained by ion-exchange using cationic or anionic sorbents. Weak acids
can be extracted from aqueous solutions of high pH when they are ionized, and weak bases from
aqueous solutions of low pH when they are protonated (cf. solvent extraction, p. 59). Alternatively, by
supressing ionization through pH control, extraction by C 18 or C 8 sorbents may be possible. Sorbents of

intermediate polarity, such as cyanopropyl and aminopropyl modified silicas may have different
selectivities to non-polar and polar sorbents.

Solid Phase Extraction Formats

Syringe-barrel Cartridges

Currently, most SPE is carried out using a small packed bed of sorbent (25–500 mg) contained in a
cartridge made from a polypropylene syringe barrel, the sorbent being retained in position by